With the recent development of mobile communication devices and portable electronic devices, demand is greatly increasing for power sources for those devices such as batteries and capacitors. In particular, rechargeable lithium secondary batteries are widely used as a power source for portable electronic devices because they have high electromotive force and high energy density and are capable of operating repeatedly.
As the miniaturization and weight reduction of portable electronic devices advance, however, the demand is increasingly focused on a battery with high energy density. Accordingly, a novel electrode material with high energy density is desired. Under such circumstances, material development has actively been carried out with the objective of creating an electrode material with high energy density and high power density that can directly contribute to the production of a high energy and high power density battery.
In order to produce a more lightweight battery with high energy and high power density, the use of an organic compound as an electrode material has been investigated. Organic compounds are as light as a specific gravity of about 1 g/cm3, which is lighter than oxides currently used as a material for lithium secondary batteries such as lithium cobaltate. With the use of an organic material, it is possible to produce a lightweight battery with high capacity.
U.S. Pat. No. 5,833,048 and Japanese Patent No. 2,715,778 propose a secondary battery using an organic compound with disulfide bonds as an electrode material. This organic sulfur compound is most simply represented by the formula: M+-−S—R—S−M+, where R represents an aliphatic or an aromatic organic group, S represents sulfur, and M+ represents a proton or metal cation. The compound bonds together by an S—S bond through an electrochemical oxidation reaction to give a polymer with a structure of M+-−S—R—S—S—R—S—S—R—S−-M+. Thus-produced polymer returns to the original monomers by an electrochemical reduction reaction. This reaction is applicable to a charge/discharge reaction in secondary batteries.
U.S. Pat. No. 5,523,179 proposes to use elementary substance sulfur as an electrode material.
In either case, however, the problem arises that the materials have low cycle life characteristics although it is possible to achieve high capacity. This is because a recombination frequency is low in the dissociation and recombination of a disulfide bond during the oxidation-reduction reaction of a sulfur-based material. Low recombination frequency means that all reactive portions can not react even if the material theoretically has high energy density. Therefore, it cannot actually be said that the compounds of the above related art examples are materials having high energy density.